1. Field of the Invention
This invention generally relates to a water based ink exhibiting a metallic gloss (sometimes referred to as metallic color) which is capable of being used for directly filling a cylindrical chamber of a ball-point pen. More particularly, the present invention relates to a water-based ball-point pen grade ink which may possess a metallic gloss of gold, silver, etc.
2. Description of Related Art
Inks possessing a metallic gloss have been disclosed in, for example, JP-A-61-123,684, JP-A-63-95,277, and JP-A-04-126,782. However, these inks exhibit such problems as they invariably have a very low viscosity and are often incapable of preventing settling of metallic powder having a high specific gravity. Therefore, when the inks of the prior art are used, vibration or agitation is generally required prior to use.
In JP-A-07-145,339, in one example, a polysaccharide such as guayule gum or xanthan gum (which can provide a shear thinning property to the ink) is disclosed as being added to an ink in order to prevent settling of metallic powder. The metal ions that dissociate from the metallic powder react with the natural polysaccharide (a macromolecular viscosity enhancer) which eventually induces serious addition to the viscosity of the ink by aging, that is, the ink becomes more viscous over time.
A natural polysaccharide generally has a smaller yield value than a synthetic macromolecular substance. Thus, when a synthetic macromolecular substance is used as a viscosity enhancer, the settling of a metallic power cannot be completely prevented, even when the viscosity of the ink is set at a high level and the specific gravity of the metallic powder exceeds the yield value. Yield value is generally understood as the value of sheer force required to transfer an ink from a gel state into a sol state. Further, even when a natural macromolecular substance is used as a viscosity enhancer, since the yield value thereof is small, the settling of the metallic powder cannot be inhibited unless the viscosity of the ink is extremely high. Setting the viscosity at high level requires an increase in the amount of the natural macromolecular substance added. Most macromolecular viscosity enhancers, as known in the art, exhibit a film-forming property due to the evaporation of water from water-based inks. The film-forming property of macromolecular viscosity enhancers and the copiousness of the ink lead to solidification of the ink at the tip of the pen, thus seriously inhibiting smooth flow of the ink from the pen.